When wounded, many trees exude a thick, viscous sap. It can look a bit like honey. It congeals and dries out, sealing the wound. Some trees are particularly prone to this, and the dried and hardened sap forms hard blobs of a transparent golden yellow substance. Sometimes, as the sap is still oozing from the tree, an insect might land on it, tempted by the sweet smell. This spells doom for the poor creature, as the sap is sticky. The insect can get encased in the flowing sap, and preserved inside as it hardens. The hardened sap is the substance we call amber.

(For bonus points today, without reading any further, try now to guess the topic of today's discussion.)

Amber is a beautiful substance, radiant and warm. Unlike metals or stones, it is organic and, because of its heat conducting properties, does not feel cold to the touch. Pieces of amber have been used as decorations for thousands of years.

But amber has another interesting property. If you rub it on some wool you can start to hear a faint crackling sound. And then if you hold the amber near some tiny pieces of paper, the paper will leap up into the air and stick to the amber. The Ancient Greeks knew about this. The effect, as we recognise today, is that of static electricity. Our modern word electricity comes from the Ancient Greek word for amber: elektron. (I've discussed this before, when setting up to talk about chemical bonding. Today we're going in a different direction.)

Wool.

Now we know that matter is made of atoms, each with a nucleus of protons and neutrons, surrounded by a cloud of electrons. When two compatible materials are rubbed together, one tending to give up electrons easily (like wool) and one tending to attract electrons (like amber), the energy supplied by the rubbing can strip electrons off the atoms of one substance and deposit them onto the other. The result is that one object (the wool) ends up with an overall positive electric charge, while the other (the amber) becomes negatively charged.

This imbalance of electric charge causes certain effects. Positive and negative electric charges attract one another, so the wool and amber tend to become attracted to one another. But if you pull them apart, each object starts to attract other objects around it, such as little pieces of paper. This happens because the electrons in nearby objects shift around, moving away from negative charges and towards positive charges. Although each piece of paper remains electrically neutral overall, its charges are redistributed so that the part nearest the amber is now positive and so becomes attracted to the negatively charged amber. The result is that even though little bits of paper are normally electrically neutral, they are attracted to both negatively and positively charged objects.

The attractive force is also present with many other objects, but it is weak, so only very light objects like pieces of paper or hair strands actually move towards the charged object. You can notice it as the static cling of light clothing objects fresh out of the dryer, or styrofoam beads sticking to a cat.

Electrical sparks.

The other phenomenon you commonly find with static electricity is the crackling sound as you pull stuck objects apart, or the small shock you experience when you yourself carry a static charge and you touch a metal object or another person. The cause is the same in each case: a moderately large electron imbalance between objects close to one another. The rubbing of one suitable material against another strips electrons off one object and deposits them on the other. But the charge cannot stay in this imbalance if there is a way for it to rebalance itself again. The atoms stripped of electrons are now positively charged and so attract electrons to them, while the atoms clinging to extra electrons are negatively charged and so repel electrons. If separated by a large gap of air, the electrons cannot escape back to the positively charged atoms. But if the gap is small, the electrical energy can breach the air gap and the electrons can fly back. When they do, they cause a brief spark.

Static electricity is one thing: when there is an imbalance in electrical charges. But this spark is different: it is the movement of electric charge. It is what we call an electric current, by analogy with a current of water. The electrons flow from one place to another.

An electric current is driven by an imbalance in electric charge. When one place is more positively or negatively charged than another place, then electrons in the more negative of the two places feel a force pulling them from there towards the more positively charged place. There is the potential for the electrons to move. This tendency for electric charges to move when charges are unbalanced is called, reasonably enough, an electric potential. Although you might not have heard of "electric potential" before, you are probably familiar with the units of measurement. Electric potential is measured in the units known as volts, and the difference in the amount of electric potential between two points in space is commonly known as the voltage between those points.

Measuring the electric potential difference of a mains power socket. DO NOT TRY THIS, it is EXTREMELY DANGEROUS. Public domain image.

Similar to gravity, electrical potential has the capability of bestowing energy. But whereas gravity works on mass, electrical potential works on electric charge. And similarly to how a mass suspended in a region with gravity has a gravitational potential energy, an electric charge suspended in a region of electric potential has an electric potential energy. Both of these forms of potential energy can be converted into energy of motion (kinetic energy), of the object moving from a region of high potential energy to a region of lower potential energy. In the case of a mass near the Earth's surface, this means falling downwards. But in the case of an electric charge, the potential can be directed in any direction, depending on the arrangement of the positive and negative charges around it.

But electric charges are not free to move anywhere, in the same way that a massive object is not free to fall if there is something underneath it, blocking the way. Most things resist the passage of electric charges through them, including air. Usually, here on Earth's surface, the movement of electric charge means the movement of electrons. More broadly speaking, this is not always the case, as the movement of charged atoms, or ions, also counts. Charged atoms or other charged particles emitted by objects like stars, pulsars, and black hole accretion discs stream through space relatively unhindered, and these certainly count as movement of electric charge, but I don't want to concentrate on that today. Radioactive materials also emit highly energetic charged particles, which shoot through the air like tiny bullets. But putting these energetic processes aside for now, I want to concentrate on more sedate drifts of electrons, driven by electric potentials.

You can find electric potential differences readily in your home. Pick up a battery. The positive and negative terminals are at different electric potentials. The active and neutral contacts buried within your mains power sockets are also at different electric potentials. The potential difference, or voltage, of a battery is fairly small, the order of a few volts. The potential difference of a mains power socket is considerably larger, and so can impart a larger amount of energy. A battery is fairly harmless, and you can touch the terminals without a problem. But mains power carries enough energy to cause electrical damage to your body, which is why the metal contacts are shielded from prying fingers. (This is not the entire story - there is also a difference between the direct current (DC) voltage of a battery and the alternating current (AC) voltage of a mains socket, but that's for another day.)

The fact that battery and power socket terminals are metal, while things designed to protect you from the dangers of electricity are not is no coincidence. As stated above, most things resist the flow of electric charges, in this case electrons. But there are some materials which allow electrons to move easily through them. These materials have the property that their atoms contain full shells of electrons (I discussed atomic electron shells in #3276), plus a small number of extra electrons, to balance the number of protons in their nuclei. The few electrons not in full shells are rather loosely held by the atoms, and can slip freely across to neighbouring atoms of the same material. Get enough atoms like this together, and you form a bulk material which has billions upon billions of "loose" electrons, all sort of sloshing around amongst the underlying solid structure of the atoms. We call such materials metals. They include pretty much what you might expect: iron, copper, gold, silver, aluminium, tin, lead, and so on.

Not possible without electric potential.

If you apply an electric potential difference, or a voltage, across a gap of air, or a piece of wood, or plastic, or most other substances, nothing happens. This is because the material resists the flow of electrons. But if you apply the same voltage across a piece of metal, the loose electrons in the metal can move away from the more electrically negative region, towards the more positive region. In other words, you set up an electric current - an overall motion of electrons in a certain direction. If you have nothing but a chunk of metal in between your voltage terminals, this is a great way to drain your battery of energy really quickly. Or if you do this with a mains power socket—WARNING: do NOT try this, it is EXTREMELY DANGEROUS—you will cause an enormous current surge that will blow your home's circuit breakers if you're lucky, and kill you and set fire to your home if you are unlucky.

But you can put things other than just bare metal between electrical terminals. Some things you might want to try: light bulbs, television sets, toasters, washing machines, refrigerators, computers. Our electrical appliances are designed to connect to a source of electric potential difference and use the energy given to the electrons in them to do useful things: give off light or heat, or sound, or move things around, or perform other, more complex tasks for us. Different components within these devices variously restrict or switch the flow of electrons within them. The results can be amazingly complex, and depend on the details of interactions between moving charges and atoms.